Method and apparatus for performing fast power control in a mobile communication system

ABSTRACT

A method and apparatus for controlling transmission power levels in a mobile communication system. The method provides for a closed-loop power control method. A mobile station provides information on the quality of the signal received from the base station, and the base station responds by adjusting the power allocated to that user in a shared base station signal. The transmission power is adjusted initially by a large increment and then ramped down at an increasingly decreasing rate. The mobile station also provides information to the base station as to its relative velocity and the base station adjusts its transmission power in accordance with this velocity information.

CROSS REFERENCE INFORMATION

This application is a continuation of application Ser. No. 09/454,926,filed Dec. 3, 1999, now U.S. Pat. No. 6,317,587, which is a continuationapplication of Ser. No. 08/958,882, filed Oct. 27, 1997, now U.S. Pat.No. 6,035,209, which is a file wrapper continuation application of Ser.No. 08/414,633, filed Mar. 31, 1995, now abandoned.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communication systems. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for controlling transmission power in a mobilecommunication system.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques isone of several techniques for facilitating communications in which alarge number of system users are present. Other multiple accesscommunication system techniques, such as time division multiple access(TDMA) and frequency division multiple access (FDMA) are known in theart. However, the spread spectrum modulation technique of CDMA hassignificant advantages over these modulation techniques for multipleaccess communication systems. The use of CDMA techniques in a multipleaccess communication system is disclosed in U.S. Pat. No. 4,901,307,entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USINGSATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of thepresent invention, of which the disclosure thereof is incorporated byreference herein. The use of CDMA techniques in a multiple accesscommunication system is further disclosed in U.S. Pat. No. 5,103,459,entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMACELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the presentinvention, of which the disclosure thereof is incorporated by referenceherein.

CDMA by its inherent nature of being a wideband signal offers a form offrequency diversity by spreading the signal energy over a widebandwidth. Therefore, frequency selective fading affects only a smallpart of the CDMA signal bandwidth. Space or path diversity is obtainedby providing multiple signal paths through simultaneous links from amobile user through two or more cell-sites. Furthermore, path diversitymay be obtained by exploiting the multipath environment through spreadspectrum processing by allowing a signal arriving with differentpropagation delays to be received and processed separately. Examples ofpath diversity are illustrated in U.S. Pat. No. 5,101,501 entitled“METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN ACDMA CELLULAR TELEPHONE SYSTEM”, and U.S. Pat. No. 5,109,390 entitled“DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assignedto the assignee of the present invention and incorporated by referenceherein.

A method for transmission of speech in digital communication systemsthat offers particular advantages in increasing capacity whilemaintaining high quality of perceived speech is by the use of variablerate speech encoding. The method and apparatus of a particularly usefulvariable rate speech encoder is described in detail in U.S. Pat. No.5,414,796 having a priority date of Jun. 11, 1991, entitled “VARIABLERATE VOCODER”, assigned to the assignee of the present invention, whichis incorporated by reference herein.

The use of a variable rate speech encoder provides for data frames ofmaximum speech data capacity when said speech encoding is providingspeech data at a maximum rate. When a variable rate speech coder isproviding speech data at a less that maximum rate, there is excesscapacity in the transmission frames. A method for transmittingadditional data in transmission frames of a fixed predetermined size,wherein the source of the data for the data frames is providing the dataat a variable rate is described in detail in U.S. Pat. No. 5,504,773,having a priority date of Jan. 16, 1992, entitled “METHOD AND APPARATUSFOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assigneeof the present invention, which is incorporated by reference herein. Inthe above mentioned patent application a method and apparatus isdisclosed for combining data of differing types from different sourcesin a data frame for transmission.

In frames containing less data than a predetermined capacity, powerconsumption may be lessened by transmission gating a transmissionamplifier such that only parts of the frame containing data aretransmitted. Furthermore, message collisions in a communication systemmay be reduced if the data is placed into frames in accordance with apredetermined pseudorandom process. A method and apparatus for gatingthe transmission and for positioning the data in the frames is disclosedin U.S. Pat. No. 5,659,569, having a priority date of Mar. 5, 1992,entitled “DATA BURST RANDOMIZER”, assigned to the assignee of thepresent invention and incorporated by reference herein.

A useful method of power control of a mobile in a communication systemis to monitor the power of the received signal from the mobile stationat a base station. The base station in response to the monitored powerlevel transmits power control bits to the mobile station at regularintervals. A method and apparatus for controlling transmission power inthis fashion is disclosed in U.S. Pat. No. 5,056,109, entitled “METHODAND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULARTELEPHONE SYSTEM”, assigned to the assignee of the present invention, ofwhich the disclosure thereof is incorporated by reference herein.

In a communication system that provides data using a QPSK modulationformat, very useful information can be obtained by taking the crossproduct of the I and Q components of the QPSK signal. By knowing therelative phases of the two components, one can determine roughly thevelocity of the mobile station in relation to the base station. Adescription of a circuit for determining the cross product of the I andQ components in a QPSK modulation communication system is disclosed inU.S. Pat. No. 5,506,865, entitled “PILOT CARRIER DOT PRODUCT CIRCUIT”,assigned to the assignee of the present invention, the disclosure ofwhich is incorporated by reference herein.

In an alternative continuous transmission strategy, if the data rate isless than the predetermined maximum, then the data is repeated withinthe frame such that the data occupies the full capacity of the dataframe. If such a strategy is employed, power consumption andinterference to other users may be reduced during periods of datatransmission at less than the predetermined maximum by reducing thepower at which the frame is transmitted. This reduced transmission poweris compensated by the redundancy in the data stream and can offerbenefits in range for a fixed maximum transmission power.

A problem that is encountered in controlling transmission power in thecontinuous transmission strategy is that the receiver does not know thetransmission rate a priori and so does not know the power level thatshould be received. The present invention provides a method andapparatus for controlling transmission power in a continuoustransmission communication system.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and apparatus forclosed loop transmission power control in a communication system. It isan object of the present invention to provide timely power control thatis necessary to provide robust communication link quality under fadingconditions.

Further, it should be noted that power control techniques are presentedin the exemplary embodiment in a spread spectrum communication system,however, the methods presented are equally applicable for othercommunication systems. Also, the exemplary embodiment used for thecontrol of transmission power in transmissions from a base station to aremote or mobile station may be applied to the control of transmissionpower in transmissions from a remote or mobile station to a basestation.

In the exemplary embodiment, a base station transmits packets of data toa mobile station. The mobile station receives, demodulates and decodesthe received packet. If the mobile station determines that the receivedpacket cannot be reliably decoded, it sets the normally ‘0’ qualityresponse power control bit to ‘1’ indicating the situation to the basestation. In response, the base station increases the transmission powerof the signal to the mobile station.

In the exemplary embodiment of the present invention, when the basestation increases its transmission power it does so with a relativelylarge step in transmission power which is assumed to be more thanadequate under most fading conditions. The base station then decreasesthe transmission power level at an exponentially decreasing rate as longas the quality response power control bits remain at ‘0’. In analternative embodiment, the base station responds to a request from themobile station for additional signal power by increasing the signalpower incrementally.

In an improved embodiment of this power control system, the base stationwill determine whether the error reported by the mobile station was of arandom nature in which case it will immediately begin ramping down thetransmission power or whether the error was an error resulting from agenuine fading condition. The base station distinguishes errors of arandom nature from those of a prolonged nature by examining the patternsof power control bits sent by the mobile station. If the pattern ofpower control request signals and sends a 1-bit quality power controlresponse in the packets it transmits back to the base station andindicates that there is a new fading condition present in thepropagation path, then the base station will refrain from decreasing thetransmission power.

One of the identified sources of sudden changes in the propagation pathof a mobile station is a change in velocity relative to the position ofthe base station. That is, if the velocity towards the mobile station oraway from the mobile station is changing. In the present invention, themobile station determines that the velocity relative to the base stationis changing, and if necessary, sets the power control bits to requestadditional power from the base station to accommodate the change invelocity.

In a first exemplary embodiment, the mobile station is equipped with amotion sensor which may operate off of information from the speedometeror tachometer in the case of an automobile based mobile station. Themobile station then generates the power control signal in accordancewith the signal from the motion sensor.

In a second exemplary embodiment, the mobile station may sense a shiftin the received signal from the base station in order to sense motion.In the exemplary embodiment, the mobile station determines the changesin relative velocity by measuring the Doppler shift in the receivedpilot signal.

In a third exemplary embodiment, the base station determines thepresence of motion by sensing changes in the incoming signal and adjuststransmission power in accordance with these changes.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is an illustration of an exemplary mobile telephone system;

FIG. 2 is an illustration of the apparatus of the present invention; and

FIG. 3 is an illustration of a curve illustrating the delay timeentailed in a closed loop power control system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the present invention is illustrated in anexemplary implementation in a mobile communication system forcontrolling the power of transmissions between base station 4 and mobilestation 6. Information may be provided to and from a public switchedtelephone network (PSTN) to system controller and switch 2, or may beprovided to and from controller and switch 2 by another base station ifthe call is a mobile station to mobile station communication. Systemcontroller and switch 2, in turn, provides data to and receives datafrom base station 4. Base station 4 transmits data to and receives datafrom mobile station 6.

In the exemplary embodiment the signals transmitted between base station4 and mobile station 6 are spread spectrum communication signals, thegeneration of the waveforms of which are described in detail in theabove mentioned U.S. Pat. No. 4,901,307 and U.S. Pat. No. 5,103,459. Thetransmission link for communication of messages from mobile station 6and to base station 4 is referred to as the reverse link and thetransmission link for communication of messages from base station 4 andto mobile station 6 is referred to as the forward link. In the exemplaryembodiment, the present invention is used to control the transmissionpower of base station 4. However, the methods of power control of thepresent invention are equally applicable to controlling the transmissionpower of mobile station 6.

Referring to FIG. 2, base station 50 and mobile station 30 areillustrated in block diagram form showing the apparatus for providingcontrol of the transmission power of base station 50 of the presentinvention. If a communication link degrades, then the link quality canbe improved by increasing the transmission power of the transmittingdevice. In the exemplary embodiment of controlling transmission power ofbase station 50, some of the methods for determining that thetransmission power of base station 50 should be increased include:

(a) mobile station detection of frame errors on forward link;

(b) mobile station detects that received power is low on forward link;

(c) mobile station to base station range is large;

(d) mobile station location is poor;

(e) mobile station change in velocity; and

(f) mobile station detects that received power on pilot channel is lowon forward link.

Conversely, some of the methods for determining that the transmissionpower of base station 50 should be decreased include:

(a) mobile station quality responses to the base station show a lowframe error rate for the forward link;

(b) mobile station detects that received power is high on forward link;

(c) base station to mobile station range is low;

(d) mobile station location is good; and

(e) mobile station detects that received power on forward link pilotchannel is high.

When base station 50 detects a need to modify the transmission power ofthe forward link, control processor 58 sends a signal specifying amodified transmission power to transmitter (TMTR) 64. The modified powersignal may simply indicate a need to increase or decrease thetransmission power or it may indicate an amount to change the signalpower or it may be an absolute signal power level. In response to themodified power level signal, transmitter 64 provides all transmission atthe modified transmissions power level.

It should be noted that data source 60 may source modem, facsimile orspeech data. Data source 60 may be a variable rate source that variesits transmission rate on a frame to frame basis throughout thetransmission or may be able to vary rates only upon command. In theexemplary embodiment, data source 60 is a variable rate vocoder. Thedesign and implementation of a variable rate speech vocoder aredescribed in detail in the aforementioned application Ser. No.08/004,484, now U.S. Pat. No. 5,414,796. The output from data source 60is encoded by encoder 62 and input to traffic modulator 63 formodulation and input to transmitter 64. Also input to pilot transmitter65 is a synchronous pilot signal for transmission.

A need for modification of the transmission power may be indicated byany one of the conditions enumerated above or by any combination ofthose conditions. If the method of power control is based upon aposition related effect such as range or mobile station location, thenan external signal (LOCATION) is provided to control processor 58 ofbase station 50 indicative of the location condition. The rangecondition can be detected by base station 50. In an alternativeembodiment the range condition can be detected by mobile station 30 andtransmitted to base station 50. In response to the detected rangecondition control processor 58 in base station 50 generates a controlsignal for modifying transmission power of transmitter 64.

In a closed loop power control implementation, power control signals areprovided from mobile station 30 to base station 50. Mobile station 30may determine the power control signal in accordance with received poweror alternatively in accordance with the detection of frame errors. Thepresent invention is equally applicable to any link quality factors.

If the link quality factor used is received power, then the signal frombase station 50 received at mobile station 30 by antenna 38 and providedto receiver (RCVR) 42 which provides an indication of the received powerto control processor 46. If the link quality factor used is thedetection of frame errors, then receiver 42 downconverts and amplifiesthe signal providing the received signal to traffic demodulator 43. Ifthe traffic signal is accompanied by a pilot signal in order to providefor coherent demodulation then the received signal is also provided topilot demodulator 45 which demodulates the signal in accordance with apilot demodulation format and provides a timing signal to trafficdemodulator 43. Traffic demodulator 43 demodulates the received signalin accordance with a traffic demodulator format. In the exemplaryembodiment, traffic demodulator 43 and pilot demodulator 45 are CDMAspread spectrum demodulators, the design of which is described in theaforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459. Trafficdemodulator 43 provides the demodulated signal to decoder 44. In a firstexemplary embodiment, decoder 44 performs error—detection decoding todetermine if errors have occurred. Error detection/correction decoderssuch as the Viterbi trellis decoder are well known in the art. In analternative embodiment, decoder 44 decodes the demodulated signal andthen re-encodes the decoded signal. Decoder 44 then compares there-encoded signal with the demodulated signal to obtain an estimate ofthe channel symbol error rate. Decoder 44 provides a signal indicatingan estimated channel symbol error rate to control processor 46.

Control processor 46 compares the received power or estimated channelsymbol error rate referred to generically as the link quality factoragainst a threshold or set of thresholds which may be static or varying.Control processor 46, then provides the power control information toeither encoder 34 or power control encoder (P.C. ENC.) 47. If the powercontrol information is to be encoded into the data frame, then the powercontrol data is provided to encoder 34. This method requires that anentire frame of data be processed before transmitting the power controldata, then encoded traffic data containing power control data areprovided to transmitter (TMTR) 36 through modulator 35. In analternative embodiment, the power control data may simply overwriteportions of the data frame or may be placed in predetermined vacantpositions in the transmission frame. If the power control dataoverwrites traffic data, then this may be corrected by forward errorcorrection techniques at base station 50.

In implementations that process a full frame of data before providingthe power control data, the delay waiting for a full frame to beprocessed is undesirable in fast fade conditions. The alternative is toprovide the power control data directly to modulator 35 where it may bepunctured into the outgoing data stream. If the power control data istransmitted without error correction coding then control processor 46outputs the power control data directly to modulator 35. If errorcorrection coding is desired for the power control data, controlprocessor 46 outputs the power control data to power control encoder 47which encodes power control data without regard to the outgoing trafficdata. Power control encoder 47 provides the encoded power control signalto modulator 35 which combines the encoded power control signal with theoutgoing traffic data provided from data source 32 through encoder 34 tomodulator 35. Transmitter 36 upconverts and amplifies the signal andprovides it to antenna 38 for transmission to base station 50.

The transmitted signal is received at antenna 52 of base station 50 andprovided to data receiver (RCVR) 54 where it is downconverted andamplified. Receiver 54 provides the received signal to demodulator 55which demodulates the received signal. In the exemplary embodiment,demodulator 55 is a CDMA spread spectrum demodulator which is describedin detail in the aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459.If the power control data is encoded within a frame of traffic data,then the traffic and power control data is provided to decoder 56.Decoder 56 decodes the signal and separates the power control signalfrom the traffic data.

If, on the other hand the power control data is not encoded with a fullframe of data but rather punctured into the transmission stream of data,then demodulator 55 demodulates the signal and extracts the powercontrol data from the incoming data stream. If the power control signalis not encoded then demodulator 55 provides the power control datadirectly to control processor 58. If the power control signal is encodedthen demodulator 55 provides the encoded power control data to powercontrol decoder (P.C. DEC.) 55. Power control decoder 55 decodes thepower control data and provides the decoded power control data tocontrol processor 58. The power control signal is provided to controlprocessor 58, which in accordance with the power control signal providesa control signal to transmitter 64 indicative of a modified transmissionpower level.

One of the inherent problems with closed-loop power control systems is arelatively slow response time, relative to an open-loop power controlsystem. For example, in a closed-loop power control system, when basestation 50 transmits a frame at an insufficient transmission energy tomobile station 30, mobile station 30 receives and decodes the frame,determines whether the frame is in error, prepares a power controlmessage indicating the frame error, then transmits the power controlmessage to base station 50, which decodes the frame, extracts the powercontrol message and adjusts the transmission power of transmitter 64.This results in a four frame time lag before correction is apparent atmobile station 30. Thus, if the propagation path has deteriorated, fourconsecutive frames would be transmitted at the same insufficient frameenergy before a frame is transmitted at the adjusted frame energy. Inthis delay period the fading condition may have substantially improvedor deteriorated.

The following are methods by which to improve the responsiveness of aclosed loop power control system. In a first exemplary embodiment of thepresent invention, the base station assumes the worse case. Which isthat the propagation path has deteriorated during the four frame delayperiod. In response the base station increases the transmission energyto that user by a relatively significant amount ΔE so that theadjustment will be more than adequate to assure that the power adjustedframe will be properly received even if the propagation path hasdeteriorated in the interim. In the exemplary embodiment of a spreadspectrum communication system, this increase in power to mobile station30 causes less power to be available for other users who share theforward link. So the base station transmitter quickly reduces thetransmission energy for that user following the initial increase. In theexemplary embodiment, the base station increases the energy by a fixedamount ΔE holds that value for a delay period to verify that theincrease in transmission energy has been effective and then decreasesthe transmission energy in accordance with a predetermined piecewiselinear function as illustrated in FIG. 3.

FIG. 3 illustrates a plot of the transmission energy (E) against time.At point A the base station 50 increases the transmission energy inresponse to a power adjustment request from mobile station 30. Basestation 50 increases the transmission energy by an amount ΔE to point B.Base station 50 holds transmission at that transmission energy for apredetermined delay period then reduces the transmission energy at aswiftly decreasing rate for a predetermined number of frames to point C.At point C, if the power control message from mobile station 30 stillindicating an excess of transmission energy, base station 50 continuesto decrease the transmission energy, however, the rate of the decreaseis less. Again, base station 50 decreases at this intermediate rate ofdecrease for a predetermined number of frames until point D. At point Dthe rate of decrease is again reduced to a final decreasing rate atwhich the transmission energy will continue to be decreased until basestation 50 reaches some minimum value or it is alerted again by anotherpower adjustment request from mobile station 30, which occurs at pointE. This power adjustment continues throughout the duration of theservice provided.

Base station 50 performs the adjustment of the transmission energy withknowledge that after the transmission energy has been increased therewill be a delay before the received power control information willreflect the change in the forward link transmission power. If thepropagation channel suddenly worsens, base station 50 will receive aseries of consecutive power control requests, and there will be a delaybefore the power adjustment requests are responsive to the change inforward link transmission energy. During this delay period, base station50 should not continue to increase the transmission energy for eachreceived power adjustment request. This is the reason that the powerlevel is held constant for a predetermined delay period as illustratedin the period following point B of FIG. 3.

It should also be noted that errors in a mobile communication systemcome in two types. Those that are random and those that are the resultof a change in the propagation path. In the exemplary embodiment, whenbase station 50 receives a power adjustment request, it increases thetransmission power by ΔE as described previously. Then it ignores thepower adjustment requests and retains the same increased power level forthe delay period. In an alternative embodiment, base station 50 adjuststhe power in accordance with each power control message. However,smaller changes would typically be used. This minimizes the impact ofrandom errors.

One of the main influences that results in changes in thecharacteristics of the propagation path between mobile station 30 andbase station 50 is motion by mobile station 30 towards or away from basestation 50. Mobile station 30 may provide base station 50 withinformation indicating that the mobile station velocity is changing orit may actually provide its velocity relative to base station 50. If themobile station is simply providing an indication that its velocity ischanging, it may provide that information as a power adjustment requestsignal in anticipation of a change in the quality of the propagationpath.

In a first embodiment, mobile station 30 may sense the change invelocity by providing a sensor to operate in accordance with a signalfrom the automobile tachometer or speedometer (not shown). In analternative embodiment, mobile station 30 determines either a change inthe mobile/base station relative velocity or absolute velocity bychanges in the received signal from base station 50. Mobile station 30may detect a change in velocity or measure the absolute relativevelocity by measuring the Doppler effect on the incoming signal frombase station 50. In an alternative embodiment, base station 50 may alsodetect a change in the mobile/base station relative change in velocityor measure the absolute relative velocity by measuring the Dopplereffect on the incoming signal from mobile station 30.

The traffic signal provided by base station 50 may be accompanied by apilot signal in order to provide for coherent demodulation of thereceived traffic signal. Use of a pilot signal is described in U.S. Pat.Nos. 4,901,307 and 5,103,459, and mobile station 30 can alternativelysense changes in the relative velocity the Doppler shift of the pilotsignal.

In a preferred embodiment, when base station 50 knows the velocity ofmobile station 30 and will vary the value of the incremental change intransmission energy, ΔE, will vary in accordance with this velocity. Thedetermination of the value of ΔE may be performed algorithmically or bya lookup table in control processor 46.

If base station 50 transmits a pilot signal along with the trafficsignal, the pilot signal can be thought of as a traffic signal thatcarries a predetermined bit stream known by mobile station 30. Mobilestation 30 demodulates the pilot channel in pilot demodulator 45 inorder to get timing information to enable mobile station 30 to performcoherent demodulation of the traffic channel. Because the pilot channeland the traffic channel are provided through similar if not identicalpropagation paths, there is a strong correlation between the strength ofthe received pilot signal and the strength of the received trafficsignal. By basing the generation of the power control signal on thepilot channel instead of the traffic channel, the delay betweenreceiving the signal transmitted from base station 50 and generation ofthe power control signal may be reduced.

Referring to FIG. 2, pilot modulator 65 provides a pilot signal totransmitter 64 and transmitter 64 of base station 50 provides the pilotsignal along with the traffic signal to antenna 52 for broadcast tomobile station 30. The transmitted signal is received at antenna 40 andprovided to receiver 42. Receiver 42 downconverts and amplifies thepilot signal and provides the received pilot signal to pilot demodulator45 generates a quality estimate of the demodulated pilot signal andprovides it to control processor 46. Control processor 46 generates apower control signal in accordance with the quality estimate of thedemodulated pilot signal and the operation proceeds as describedpreviously.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A method for controlling transmission energy of a communicationstation, comprising: determining a characteristic of a propagation pathbetween said communication station and a second communication station;increasing said transmission energy of said communication station by afirst amount in accordance with a power control step size correspondingto said characteristic of the propagation path; decreasing saidtransmission energy of said communications station from the first amountat a first predetermined rate for a period of time; and decreasing saidtransmission energy at a second predetermined rate after said period oftime.
 2. An apparatus for controlling transmission energy of acommunication station, comprising: a receiver configured to receive acharacteristic of a propagation path between said communication deviceand a second communication station; and a processor configured toincrease the transmission energy of said communication station by afirst amount in accordance with a step size corresponding to saidcharacteristic, decrease said transmission energy of said communicationstation from the first amount at a first predetermined rate for a periodof time, and decrease said transmission energy at a second predeterminedrate after said period of time.
 3. An apparatus for controllingtransmission energy of a communication station, comprising: means fordetermining a characteristic of a propagation path between saidcommunication station and a second communication station; means forincreasing said transmission energy of said communication station by afirst amount in accordance with a power control step size correspondingto said characteristic of the propagation path; means for decreasingsaid transmission energy of said communications station from the firstamount at a first predetermined rate for a period of time; and means fordecreasing said transmission energy at a second predetermined rate aftersaid period of time.